In recent years, as portable apparatuses have been downsized and multifunctionalized, there is a strong demand for increasing the capacities of the batteries as power supplies for the portable apparatuses. Carbon, which is a negative electrode active material, mainly used in lithium secondary batteries today, has a theoretical capacity of 372 mAh/g. As an active material capable of increasing the battery capacity more than carbon, there has been developed a negative electrode using an element, such as silicon, germanium, and tin, that can be alloyed with lithium. Particularly, silicon having a theoretical capacity as large as 4200 mAh/g is regarded as promising.
When a material containing an element, such as silicon, that can be alloyed with lithium is used as a negative electrode active material, the negative electrode active material expands and contracts significantly by occluding and releasing lithium therein and therefrom through charge and discharge. In contrast, a charge collector hardly expands nor contracts. Thus, repeated charges and discharges separate the negative electrode active material from the charge collector, and the negative electrode active material fails to contribute to the charge and discharge. Moreover, when the negative electrode active material expands, the charge collector is elongated beyond an elastic deformation region. As a result, the negative electrode is deformed (buckled). The deformation of the negative electrode is not preferable, either, because it leads directly to a decreased capacity of the battery.
In order to solve this problem, JP 2005-196970 A, for example, discloses that a columnar body composed of a negative electrode active material is formed on a charge collector by oblique deposition. The oblique deposition is a vapor deposition technique in which the arrangement of a vapor deposition source, a vapor deposition face, and a mask is improved so that particles coming from the vapor deposition source are incident obliquely on the vapor deposition surface. The oblique deposition allows a gap to be created between adjacent columnar bodies, thereby suppressing to some extent the deformation of the negative electrode due to the expansion and contraction of the negative electrode active material. However, the oblique deposition does not always reduce the deformation of the negative electrode to a satisfying level.
JP 2006-260928 A discloses that a negative electrode is elongated mechanically before a battery is assembled, as a technique for suppressing the deformation of the negative electrode during charge. However, in order to elongate mechanically the electrode, it is necessary to apply a tensile load to a charge collector to reach, beyond an elastic deformation region, a plastic deformation area close to a breaking strength. That is, it is difficult to control the amount of elongation, and it is difficult to apply the technique to mass production.
On the other hand, with an intent to compensate the irreversible capacity and enhance the charging and discharging cycle properties, there is known a technique of allowing a negative electrode active material layer to occlude lithium in advance before assembling a battery (JP 2004-303597 A and JP 3991966 B, for example). JP 3991966 B describes in paragraph 0014 that allowing a negative electrode active material layer to occlude lithium in advance makes it possible to alleviate the stress applied on a negative electrode collector due to the expansion and contraction of the negative electrode active material layer through charge and discharge.
However, allowing the negative electrode active material layer to occlude lithium in advance results in that lithium is present in both of the positive electrode and the negative electrode when assembling the battery. This means that the amount of lithium that can be transferred between the positive electrode and the negative electrode decreases, more specifically, that the charge and discharge capacity decreases. The charge and discharge capacity may possibly decrease significantly depending on the amount of lithium to be occluded in the negative electrode in advance. Moreover, an excess amount of lithium may possibly be precipitated on a surface of the positive electrode or the negative electrode during charge and discharge.